1. Field of the Invention
The present invention relates to a phase adjusting circuit for precisely adjusting phases of two signals having different phases to a phase difference of 90 degrees. Further, the present invention relates to a scaling signal generation circuit for generating a scaling signal having a waveform of a cyclically repeated monotonous increase using the phase adjusting circuit and a position measuring apparatus using the same.
2. Description of the Related Art
As a method of detecting a position of a mobile unit, there is a method using cyclic signals having a phase difference of 90 degrees generated when the mobile unit is detected.
In this position detecting method, as shown in FIG. 1, sensors 2s and 2c for outputting cyclic signals which change as the mobile unit moves (herein after, referred to as xe2x80x9cdetection signalsxe2x80x9d) are arranged at predetermined positions so that the phases of the detection signals become different by 90 degrees. Accordingly, two detection signals Ss and Sc ideally having the same amplitudes and a phase difference of 90 degrees are obtained from the two sensors 2s and 2c. 
The two detection signals Ss and Sc having the same amplitudes and a phase difference of 90 degrees respectively become functions of the sin xcex8 and cos xcex8 (xcex8 is a phase angle) when they are sine waves. The ratio of the two detection signals can be expressed in the form of a function of tan xcex8.
Since tan xcex8 is a function cyclically repeating a monotonous increase along with an increase of the phase angle xcex8, it can be used as a scaling signal. For example, the phase angle can be detected from 0 to 2xcfx80 from the value of tan xcex8 and the signs of sin xcex8 and cos xcex8. When equally dividing one period xcex section on the xcex8-axis into N number of fine steps, a cyclic function expressed by tan xcex8 monotonously increases in a cycle xcfx80 for every fine step having a size of xcex/n along with movement of a mobile unit, and a scaling signal is obtained for every period xcex (2xcfx80) section by referring to the signs of sin xcex8 and cos xcex8. Specifically, if a scaling signal generator 101 finds a function of tan xcex8 from the ratio of two detection signals and calculates its inverse function tanxe2x88x921 xcex8, a linear phase angle xcex8 taking discrete values equally divided into fine steps is obtained and a scaling value x is calculated from the angle xcex8 and the signs of sin xcex8 and cos xcex8. By increasing the order every period, it becomes possible to measure a position over a long distance of movement.
When using a position measuring apparatus of such a measurement principle, in a reticle alignment mechanism of a stepper or other application used installed in a semiconductor manufacturing apparatus, the required precision and resolution are high. Further, these requirements have become severer every year.
As one means for realizing high precision measurement of position, generally, as shown in FIG. 1, a conventional position measuring apparatus requires, in addition to a scaling signal generator 101 for calculating the tanxe2x88x921 xcex8 etc., a phase adjuster 102 for adjusting the phase to give the two detection signals a phase difference of exactly 90 degrees and, further, a level adjuster 103 for adjusting the amplitudes of the two signals.
In reality, however, there to no phase adjuster capable of adjusting the phase by an extremely high precision. Up until now, two sensors have been integrally formed by patterning on a substrate to improve the patterning precision at the time of formation and thereby secure the precision required for the phase difference.
On the other hand, to improve the resolution of position measurement, it is generally sufficient to increase the division number N in one period xcex section.
To increase the division number N, it is necessary to increase the number of bits of the A/D converter. However, although an 8-bit or 10-bit A/D converter (IC) is available at a low price, an A/D converter of larger bits becomes expensive. Further, a 16-bit or higher A/D converter cannot be easily obtained at the present, so there is a practical limit to the increase of the number of bits of the A/D converter.
Accordingly, the number of bits of the A/D converter Is increased to a certain extent. A further higher resolution of position measurement is attained by making the period (signal period) xcex of the detection signal shorter.
The resolution increases as the signal period xcex becomes shorter, for example, when the division number N is 400, the resolution becomes 5 xcexcm with a 2 mm xcex, 1 xcexcm with a 400 xcexcm xcex, and 0.1 xcexcm with a 40 xcexcm xcex.
If shortening the signal period xcex to increase the resolution, however, the relative positional precision between the sensors 2s and 2c becomes a problem. Namely, as explained above, the relative positions between the two sensors 2s and 2c depended on the patterning precision, but when the distance between the sensors 2s and 2c becomes shorter due to making the signal period xcex shorter, the phase deviation due to patterning error increases in the signal period xcex.
As a result, the positional precision of sensors required for accurately generating two detection signals having a 90 degree phase difference becomes stricter as the period of the detection signal becomes shorter. For example, in order to suppress the deviation of the phase difference of the sensors 2s and 2c to within 1 degree from 90 degrees, when the signal period xcex it 4 mm, the required positional precision becomes 11 xcexcmxe2x80x94still easy, while when the signal period xcex is 400 xcexcm, the positional precision becomes 1.1 xcexcm, when the signal period xcex is 40 xcexcm, and the positional precision becomes 0.11 xcexcmxe2x80x94which is very exacting. Furthermore, in actuality, since the positional deviation of a pattern must not exceed xc2xd of the resolution, a still stricter positional precision is required.
As explained above, in the position measuring apparatus of the related art, there is a trade-off between the resolution and the phase precision (positional precision of the sensors). Therefore, there has been the disadvantage that the resolution cannot be improved while maintaining a high phase precision.
An object of the present invention is to provide a phase adjusting circuit capable of providing a high phase precision regardless of the signal period, a scaling signal generation circuit, and a position measuring apparatus using the same.
According to a first aspect of the present invention, there is provided a phase adjusting circuit for adjusting phases of two input signals having different phases and generating a pair of signals having a phase difference of 90 degrees based on them, comprising an input level adjuster for adjusting at least one amplitude of said two input signals to a predetermined level; an adder for adding said two input signals after adjustment of the amplitude level and outputting a sum signal; and a subtractor for subtracting said two input signals after adjustment in amplitude level and outputting a difference signal having a phase of 90 degrees different from said sum signal.
Preferably, the circuit further comprises an output level adjuster for adjusting at least one of the amplitudes of said sum signal and said difference signal to predetermined level.
Preferably, said output level adjuster includes an amplifying circuit for amplifying a signal level of said sum signal or said difference signal.
According to a second aspect of the present invention, there is provided a phase adjusting circuit for adjusting phases of two input signals having different phases and generating a pair of signals having a phase difference of 90 degrees based on them, comprising an input level adjuster for adjusting the amplitude of at least one of said two input signals to a predetermined level and a subtractor for subtracting one input signal from the other input signal after level adjustment and outputting a difference signal having a phase difference of 90 degrees with respect to one of said input signals before level adjustment.
According to a third aspect of the present invention, there is provided a phase adjusting circuit for generating a pair of signals having a phase difference of 90 degrees based on two input signals having different phases, comprising an input level adjuster for adjusting the amplitude of at least one of said two input signals to a predetermined level and an adder for adding said input signals after level adjustment and outputting a sum signal having a phase difference of 90 degrees with respect to one of said input signals before level adjustment.
According to a fourth aspect of the present invention, there is provided a scaling signal generation circuit for generating a scaling signal having a waveform of a cyclically repeated monotonous increase based on two input signals having different phases, comprising an input level adjuster for adjusting the amplitude of at least one of said two input signals to a predetermined level; a signal processor for signal processing including calculation of at least one of addition and subtraction of said two input signals after level adjustment and generating a pair of output signals having a phase difference of 90 degrees or a single output signal having a phase difference of 90 degrees from one of said input signals; an output level adjuster for adjusting the amplitudes between said pair of output signals or between said one of input signals and said single output signal having a phase difference of 90 degrees to a predetermined level; and a scaling signal generator for generating said scaling signal based on a signal from said output level adjuster.
Preferably, said scaling signal generator receives as input two signals Exc2x7sin xcex8 and Exc2x7cos xcex8 adjusted in amplitude levels to E, where E is an adjusted amplitude, and having a phase difference of 90 degrees and finds an inverse function arctan xcex8 of tan xcex8 from the two signals so as to generate a variable xcex8 (scaling signal) cyclically repeating a monotonous increase.
According to a fifth aspect of the present invention, there is provided a position measuring apparatus for measuring a position of a mobile unit without contact, comprising a detector arranged near said mobile unit for outputting two detection signals changing as said mobile unit moves and having different phases; an input level adjuster for adjusting the amplitude of at least one of said two detection signals to a predetermined level; and a signal processor for signal processing including at least one of addition and subtraction on said two detection signals after level adjustment and generating a pair of output signals having a phase difference of 90 degrees or a single output signal having a phase difference of 90 degrees with respect to one of said detection signals.
Preferably, the apparatus further comprises an A/D converter for conversion from an analog signal to a digital signal on signals having a phase difference of 90 degrees.
Further, the apparatus further comprises an output level adjuster for adjusting the amplitudes between said pair of output signals or between said one of detection signals and said single output signal having a phase difference of 90 degrees to a predetermined level, and a scaling signal generator for generating a scaling signal based on a signal from said output level adjuster; wherein said scaling signal is converted from an analog to digital format by said A/D converter.
Preferably, the apparatus comprises a memory for storing a digital signal after said A/D conversion.
More preferably, said memory stores said digital signal after the A/D conversion in memory cells in said memory addressed by using the two signals having a phase difference of 90 degrees input to said scaling signal generator.
In the phase adjusting circuit according to the present invention, when the pair of detection signals are input to the input level adjuster from for example the detector, the amplitude of at least one detection signal is adjusted to a predetermined level in the input level adjuster. Then, the signal processor performs signal processing including calculation of at least one of addition and subtraction on the pair of detection signals adjusted in amplitude levels.
When performing both addition and subtraction in the signal processing, the sum signal and the difference signal show a format of a sin xcex8 function or a cosine function, and the phase difference of the two signals are completely to 90 degrees.
On the other hand, when performing addition or subtraction, both of a sine function term and a cosine function term appear in a formula developing the added result or the subtracted result by an addition theorem. In this case, in the level adjustment in the input level adjuster, for example, the amplitude of one of the detection signals Is adjusted to a predetermined level. At the time, the amplitude level of one detection signal is set to a value by which one of the sine function term and cosine function term is eliminated in later signal processing. As a result, the sum signal or the difference signal output from the signal processor becomes a signal having a phase exactly 90 degrees different from that of the other detection signal.
In such phase adjustment, the phase difference of the pair of signals after the phase adjustment does not change in accordance with the period length of the detection signals and always becomes 90 degrees.
The scaling signal generation circuit according to the present invention comprises an output level adjuster and a scaling signal generator in addition to the above phase adjusting circuit. The signals having a phase difference of 90 degrees output from the phase adjusting circuit are, after being adjusted in amplitude levels in the output level adjuster, input to the scaling signal generator. In the scaling signal generator, an inverse function of tan xcex8 is obtained from the signals having a phase difference of 90 degrees and a scaling signal is obtained by using the same.
The position measuring apparatus according to the present invention comprises a detector, an A/D converter, memory, etc. in addition to the above scaling signal generation circuit. The scaling signal is input to the A/D converter where it is converted from an analog to digital format. The scaling signal after the A/D conversion (digital value) is stored in the memory in advance. Then, every time the detection signals change along with movement of the mobile unit, a digital value indicating a position of the mobile unit is output from the memory.
In the position measuring apparatus, a pair of signals having a phase difference of 90 degrees is obtained by signal processing. The phase difference does not change along with the period of the detection signals. Accordingly, it is possible to improve the resolution by shortening the wavelengths of the detection signals while maintaining a high phase precision.